Varistors have non-linear resistance properties in which resistances thereof rapidly decline when voltages input thereto exceed predetermined levels and are widely used as electronic components for suppressing surge voltages generated commonly in electronic devices. An encapsulated varistor including a varistor element encapsulated in an external package made of epoxy resin or the like is used in order to increase insulation and protection from mechanical impacts, heat, moisture, chemicals, and others.
FIG. 6 is a cross sectional view of a conventional encapsulated varistor. The varistor includes a varistor element and an external coating of epoxy resin which encloses the varistor element. A voltage-non-linear resistor 21 is fabricated by, for example, molding and sintering to have a disk shape of a powder mixture which comprises ZnO, Bi2O3, and others. Electrodes 22 are formed by baking on both sides of the non-linear resistor 21. The electrodes 22 is connected by solder 24 to lead terminals 23, thus providing the varistor element. The non-linear resistor 21, the electrodes 22, the solder 24, and portions of lead terminals 23 are encapsulated in an external coating 25 of epoxy resin.
When the encapsulated varistor is loaded with a voltage exceeding its rated level, the voltage-non-linear resistor is broken down at a non-ohmic interface having a low resistant region remain unchanged, thus having its overall resistance decrease. When the encapsulated varistor is connected in series to a protective fuse for protection of a circuit, the decreasing resistance allows a large electric current to flow continuously across it, hence heating up the broken non-ohmic interface of the voltage-non-linear resistor within a short period of time before the fuse is melt down. Accordingly, since the voltage-non-linear resistor, the electrodes, lead wires, and the external coating of epoxy resin are heated up, fused, and decomposed, they produce a considerable volume of gaseous substance. The gaseous substance breaks the external coating and permit melting fragments to leak.